A fuel cell formed of a fuel cell stack is generally known. In the fuel cell stack, plural unit cells, for example, several hundred unit cells, are stacked. The unit cell has a laminated structure in which a separator is laminated on each side of a plate-shaped membrane electrode assembly (MEA). The MEA has a three-layer structure in which an electrolyte membrane, made of ion-exchange resin or the like, is held between a pair of electrodes. The pair of electrodes is made of a positive electrode (air electrode, cathode) and a negative electrode (fuel electrode, anode). In such a fuel cell, for example, a fuel gas is supplied to a gas flow path which faces a gas diffusion electrode at the side of the fuel electrode, and an oxidant gas is supplied to a gas flow path which faces a gas diffusion electrode at the side of the air electrode. As a result, electrochemical reactions occur, and electricity is generated.
In order to stabilize the electrochemical reactions, the MEA is desirably humidified. For example, a fuel cell system is disclosed in Japanese Unexamined Utility Model Application Publication No. 61-3671. In this fuel cell, water is generated by the power generation and is then supplied to a fuel gas in a fuel gas flow path as water vapor. Therefore, an off-gas, which is exhausted from an anode and has increased water vapor partial pressure, is used as a humidifying gas to humidify a fuel gas before it is used.
Recently, in accordance with a trend toward increasing performance of fuel cells, the thickness of the MEA has been decreased. As a result, water, which is generated by the electrochemical reaction and comes out to the side of the air electrode, tends to move to the side of the fuel electrode. Therefore, when the fuel gas is humidified, the fuel electrode is excessively humidified, whereby a phenomenon called “flooding” occurs and prevents contact of the fuel with the fuel electrode. In contrast, it is known that there may be a case in which the electrochemical reaction is not much affected even when the air electrode is excessively humidified. Accordingly, a technique of humidifying the oxidant gas has recently become more important than the technique of humidifying the fuel gas.
A conventional fuel cell system for automobiles, which humidifies an oxidant gas, is disclosed in Japanese Patent Application Laid Open No. 6-132038, for example. This fuel cell system has a humidifier including spaces that are divided by a moisture permeable membrane. By supplying an unused dry oxidant gas to one of the spaces and by supplying a humidified off-gas of the oxidant gas to the other space, moisture migrates from the off-gas to the oxidant gas through the moisture permeable membrane.
In this technique, moisture migrates through the moisture permeable membrane with an approximately flat shape due to contact of the off-gas with the oxidant gas from each side thereof. Therefore, the contact area is small, whereby the moisture migration does not overtake the continuous supply of the oxidant gas, and humidifying efficiency is low.
In order to overcome this problem, techniques are disclosed in, for example, Japanese Patent Applications Nos. 2002-147802, 2004-311287, 2005-40675, and 2007-323982. In these techniques, hollow-fiber membranes are filled in a humidifier, an unused oxidant gas is supplied into insides of the hollow-fiber membranes, and an off-gas is supplied so as to contact outside walls of the hollow-fiber membranes. Thus, moisture migration is performed through the hollow-fiber membranes. According to these techniques, since a lot of fine hollow-fiber membranes are filled in the humidifier, the contact area for moisture migration is extremely increased. Therefore, the humidifying efficiency is better than that in the technique disclosed in Japanese Unexamined Utility Model Application Publication No. 61-3671.
The hollow-fiber membranes swell by absorbing moisture and change in dimensions when the moisture migrates. Therefore, the hollow-fiber membranes must be filled in the humidifier by providing distances therebetween, and the hollow-fiber membranes cannot be densely filled in the humidifier. Thus, the hollow-fiber membranes have distances therebetween, and are elastically deformable. Consequently, when an off-gas is supplied to the humidifier, the off-gas pushes aside the hollow-fiber membranes at an inlet portion at which the gas flow rate is the greatest, whereby large spaces are formed. The off-gas flows through the large spaces as bypasses and thereby does not uniformly flow within the humidifier, which decreases the humidifying efficiency.
In order to overcome this problem, according to the technique disclosed in Japanese Patent Application No. 2004-311287, several hollow-fiber membranes are bounded with a rigid rod, and multiple sets are produced. By filling these sets in the humidifier, imbalance of the hollow-fiber membranes is reduced. On the other hand, according to the techniques disclosed in Japanese Patent Applications Nos. 2005-40675 and 2007-323982, a partition plate may be provided to the humidifier. The partition plate guides an off-gas flow path and prevents the hollow-fiber membranes from moving to a specific direction.
However, in the technique disclosed in Japanese Patent Application No. 2004-311287, numerous sets must be produced by binding the hollow-fiber membranes with the rigid rod. Therefore, production steps are increased, which is not preferable. In the techniques disclosed in Japanese Patent Applications Nos. 2005-40675 and 2007-323982, the imbalance of the hollow-fiber membranes is reduced compared with conventional hollow-fiber membranes. However, it is difficult to prevent the imbalance of the hollow-fiber membranes within the area divided by the partition plate. Moreover, since the partition plate cannot be formed into a completely closed structure in order to allow gas to flow, the imbalance of the hollow-fiber membranes at the open portion of the partition plate cannot be prevented.